1. Field of the Invention
The present invention relates to a method for extracting metals from aqueous waste streams, and specifically to a method for extracting and encapsulating radioactive and toxic metals from aqueous waste streams to form monolithic structures suitable for long term storage.
2. Background Information
The hydrolysis and polymerization of metal ions, such as plutonium (IV), can cause serious problems during the aqueous processing of spent fuel and nuclear wastes. These hydrous metal polymers are resistant to extraction by ion exchange and liquid/liquid extraction systems, with emulsification and interfacial crud formation during solvent extraction remaining a major roadblock. Such polymers, such as the polymeric Pu (IV) species, exhibit a marked resistance toward depolymerization, which increases with aging at high temperatures or for extended periods of time at room temperature. This aging process involves the replacement of hydroxy bridges between the metal atoms by oxo bridges. Generally, the polymer carries a net positive charge and is readily adsorbed onto a wide variety of surfaces.
Tetravalent ions, such as Zr, Hf, Ce, Th, and U, also form polymeric species. Unlike some other polymeric metal species, however, the U and Pu species are not in equilibrium with low molecular weight intermediates. The hydrolysis of these unstable moieties leads to very rapid polymerization, producing particles of colloidal dimensions that appear not to be in equilibrium with each other or with the monomeric species. Thus, reversing the metal polymerization has not, heretofore, been a feasible process approach to extraction of these species.
The partitioning of polymeric Pu (IV) in liquid/liquid extraction systems has had very limited success. Only negligible extractability of the polymer has been achieved, using dibutylcarbitol, trifluorothiophenoxyl acetone and tributyl phosphate (TBP). Each of the solvents is effective at monomeric Pu (IV) extraction. Previous methods (U.S. Pat. Nos. 4,548,790 and 4,574,072) to extract actinides and lanthanides from aqueous waste streams have utilized bidentate organo-phosphorus extractants in combination with normal paraffinic hydrocarbon diluents.
The undesirable formation of interfacial crud is a problem associated with all of the above-identified extraction systems, as the crud interferes with interfacial mass transfer of the metals being extracted.
No solvent extraction systems have been shown to successfully extract hydrous metal polymers, such as polymeric Pu(IV), giving 100 percent bulk liquid phase recovery.
There is a need for an economical and compact method to extract and encapsulate hydrous metal polymers and unhydrolyzed metal ions from aqueous solutions which are often part of specialized waste streams containing radioactive and RECRA metals. This need also extends to the treatment of secondary waste streams generated from environmental remediation and waste management activities. Such a system would provide the capability to extract and encapsulate metals whereby hazardous materials are removed from aqueous waste streams and recovered directly from an organic solvent in a single step. These hazardous metals should then be easily treated to produce stable bulk materials for safe storage by exhibiting superior resistance to physical or chemical degradation.